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EURASIP Journal on Image and Video ProcessingVolume 2009, Article ID 693274, 9 pages doi:10.1155/2009/693274 Research Article Resistivity Probability Tomography Imaging at the Castle of

EURASIP Journal on Image and Video Processing

Volume 2009, Article ID 693274, 9 pages

doi:10.1155/2009/693274

Research Article

Resistivity Probability Tomography Imaging at

the Castle of Zena, Italy

Vincenzo Compare,1Marilena Cozzolino,1Paolo Mauriello,1and Domenico Patella2

1 Department of Science and Technology for Environment and Territory, University of Molise, Via Mazzini 8,

86170 Isernia, Italy

2 Department of Physical Sciences, University Federico II, University Campus of Mt S Angelo,

80126 Naples, Italy

Correspondence should be addressed to Domenico Patella,patella@na.infn.it

Received 27 January 2009; Revised 25 June 2009; Accepted 8 October 2009

Recommended by Anna Tonazzini

We present the results of an electrical resistivity investigation performed at Castle of Zena (Castello di Zena), a 13th-century fortress located between the towns of Fiorenzuola and Piacenza in the Emilia Romagna Region (Northern Italy), in the frame of a project of restoration Dipole-dipole resistivity tomographies were planned in three areas suspected of containing buried archaeo-architectural remnants Data analysis has been made using a 3D tomography imaging approach based on the concept of occurrence probability of anomaly sources in the electrical resistivity distribution The 3D tomography has allowed three interesting anomaly

source areas to be identified in the 1-2 m depth range below ground level Subsequent excavations have brought to light a giacciara,

that is, a brickwork room for food maintenance, a furnace, and the basement of a wing of the castle destroyed in the 18th century, exactly in correspondence with the anomaly sources detected by the resistivity tomography

Copyright © 2009 Vincenzo Compare et al This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

1 Introduction

Geoelectrics is one of the most reliable prospecting tools in

the field of Cultural Heritage, thanks to the technological

and methodological developments in recent years, which

have made it a fast target-oriented method The electrical

resistivity parameter, on which the method is based, has

such a large variability so as to allow the great majority of

the structures and bodies of archaeologic and architectural

interest to be readily distinguished, in principle, from the

hosting material To enhance the resolution power of the

method, a great help is provided by the recently developed

electrical resistivity tomography (ERT) approach, which

involves the acquisition and processing of large datasets

This paper presents the results of an ERT survey,

carried out about the Castle of Zena (Castello di Zena)

Carpaneto Piacentino, in the lowland between the towns of

Fiorenzuola and Piacenza (Emilia Romagna Region, Italy)

The foundation date of the Castle of Zena are still unknown,

though the first document attesting its presence dates back to

1216 The whole complex, despite the several repairs which it has been subject to in past ages, still preserves the ancient character of a fortress of square plan, as documented in the drawing of Figure 2, dating back to 1701 and based on a land map of 1591 The southern wing of the building, that

is, the right-hand side of the map inFigure 2, is attested to have been demolished in the 18th century, thus leaving the courtyard of the castle partially exposed, as visible in the top picture ofFigure 1 On the western front, where the entry is situated, the traces of a drawbridge, replaced afterwards by

a bridge in masonry, and the ditch that surrounds the castle are visible in the lower picture ofFigure 1

The ERT survey was planned in the framework of the SOCRATES project, sponsored by institutional subjects and finalised to the study and preservation of the castle and surrounding areas The ERT survey was addressed to study the nature of the subsoil in three different zones, which the historians involved in the project suspected to contain remnants of great archaeo-architectural interest

(b)

Figure 1: The Castle of Zena, Carpaneto Piacentino (Emilia

Romagna, Italy) Views from the southern (a) and western (b) sides

2 Outline of the Geoelectrical Method

The solution for the electrical potential arising from an

electrical current flowing into the ground from a point

source of current (a grounded electrode) is the starting

theoretical point for the resistivity prospecting method In

practice, there is always a device of four electrodes used to

measure the ground resistivity: two are used for injecting

a current of intensity I and two for detecting a voltage

(potential difference) Δφ.

For all of the devices the theoretical solution is basically a

superposition of the fundamental equations for the potential

from a current point source with appropriate sign for the

current The formulae for evaluating the resistivity of the

ground are a product of the impedanceΔφ/I and a geometric

factor with the units of length which depends on the

geometry of the four electrodes However, as the resistivity

is an intrinsic property of a homogeneous material and

the subsoil is generally a complex distribution of different

materials with different resistivities, the key concept of

apparent resistivity, ρ a, is defined In simple terms, ρ a is a

volumetric average of a heterogeneous half-space, except that

the averaging is not done arithmetically but by a complex

weighting function dependent on the 4-electrode device and

how it is used

In near-surface investigations, as in the archaeological

prospection, the dipole-dipole (DD) (Figure 3) is the most

convenient 4-electrode device, since it provides a very

Figure 2: An archive document from Piacenza dating back to

1701, showing a drawing of the Castle of Zena based on land measurements of 1591

Δ

I

a a

N M

ka A

B

Figure 3: A sketch of the dipole-dipole electrode device for geoelectrical prospecting A (positive) and B (negative) are the

current electrodes to inject a current of intensityI into the ground.

M and N are the potential electrodes to measure the voltage Δφ The

dipole length isa and k =1, 2, determines the spacing between

the dipoles as an integer multiple ofa.

Zone B

Zone C

Line A20 Zone A

N

(m)

Figure 4: A sketched map of the Castle of Zena (C-shaped central light blue area) and the three ERT A, B, and C survey zones (grey areas) The red lines A20, B9, and C10 are the profiles selected to show examples of the nature of the input data in the form of 2D pseudosections

Figure 5: A-zone: cropmarks in correspondence with the giacciara

indicated in the map ofFigure 2

Log apparent resistivity (Ωm)

Distance (m)

−2

−4

Figure 6: The apparent resistivity pseudosection across the line A20

in the A-zone (red line inFigure 4)

detailed lateral bounding of vertical features The DD device

is normally used in profiling mode to map lateral as well as

depth variations of the resistivity The convention for the DD

device shown inFigure 3is that current and voltage dipole

lengths are the same, a, and the spacing between the dipoles

is an integer multiple k of a The DD apparent resistivity is

thus given by

ρ a = πak(k + 1)(k + 2) Δφ

and its values are expressed in Ohm·meters (Ωm) in the SI

system of units

To investigate the resistivity distribution along a profile,

a { ρ a } dataset is collected from a linear array of dipoles

coupled to a transmitter/receiver unit through a series of

solid state relays Measurements are realised from predefined

arrays of dipoles selected by the relays

The plotting convention is to attribute the values ofρ a

at the intersection point of two 45◦ lines descending from

the current dipole and from the voltage dipole The resulting

maps of{ ρ a }are contoured at constant (usually logarithmic)

intervals The contoured sections are called pseudosections

because they look somewhat like resistivity cross-sections

of the ground, but actually they are simply a graphical

representation of the { ρ a }dataset The vertical scale is not

depth but some function of the array spacing For simplest

geological models the relative pseudosections do have an

intuitive relationship to the actual section but mostly they do

Log apparent resistivity (Ωm)

Distance (m)

−5

−3

−1

Figure 7: The apparent resistivity pseudosection across the line B9

in the B-zone (red line inFigure 4)

Log apparent resistivity (Ωm)

Distance (m)

−5

−3

−1

Figure 8: The apparent resistivity pseudosection across the line C10

in the C-zone (red line inFigure 4)

not For a layered earth the contour lines are horizontal and rise and fall in value in the same sense as the actual resistivity, but for the case of even a single vertical contact between dissimilar resistivities the pseudosection is a complex map with no direct relationship to the actual model

A numerical inversion is used to convert measured apparent resistivity distributed along a pseudosection to electrical resistivity values displayed as a function of depth below surface

The geoelectric resistivity tomography (ERT) approach comes from taking many ρ a determinations at as many locations as possible and involves the joint inversion of many independent tests, using an algorithm to discern subtle details from differences which would otherwise not be seen

in any one test The inversion of a { ρ a } dataset collected

by the described DD profiling field technique gives rise to

a two-dimensional (2D) DD ERT If one assembles a set of parallel DD profiles, as we did in the Castle of Zena survey, the inversion of the whole { ρ a } dataset provides a three-dimensional (3D) DD ERT

Resistivity inversion is a typical nonlinear, ill-posed, and underdetermined problem [1 4] Furthermore, mainly in 3D cases, the number of the model parameters to be inverted is

so high that the large computer time required to solve the problem makes the approach almost unpractical in routine applications An efficient way of dealing with 2D and 3D inversion derives from a linearised form of the nonlinear problem One-step and iterative linear methods have been proposed; see for example, [5 8] The main advantage of such methods is that they can greatly reduce the computer time needed to generate an approximate model

−0.5 −0.25 0 0.25 0.5

Resistivity anomaly occurrence probability

Distance (m)

5

4

3

2

1

0

20

40

60

80

Distanc

e(m)

A

C

B

Figure 9: The 3D probability tomography in the three surveyed A,

B, and C zones ofFigure 4, represented with sequences of horizontal

slices at increasing depth below the ground surface The top slice is

the reference land map with the three survey zones and the sketched

plan of the castle

Following the one-step linearised strategy, a

probability-based ERT method has also been developed in more recent

years as a simple and fast anomaly source imaging tool [9

11] It has been proven to be very useful in highlighting shape

and position of the most probable sources responsible of the

ρ aanomalies detected on the free surface An outline of the

probability-based ERT method is given below, since it was

used to interpret the ERT survey performed in the Castle of

Zena area

3 Outline of the Probability Tomography

The probability tomography method consists in the analysis

of an occurrence probability function ranging between −1

and +1, defined as a normalised cross-correlation product

of the{ ρ a }dataset by a suitably digitised scanner function,

derived from the electric potential theory by a perturbation

technique under Born approximation [10]

In practice, since the source pattern generating the

observed anomalies is unknown, an elementary source of

10 20 30 40

50

60

N

30 20 10

−2

−4

(a)

110 115 120

125

25 30 35

−2

−4 (b)

85 90 95 80

5 10 15 20 25 30

−2

−4

Resistivity anomaly occurrence probability Distances (meters)

(c)

Figure 10: 3D views of the probability-based ERT representation in the A-zone (a), B-zone (b), and C-zone (c)

unitary strength is ideally used to scan the volume beneath the surveyed area, called the tomospace, and search where the sources are most probably located From the analytical point of view, this ideal process corresponds to calculating the occurrence probability function in a grid of points in the tomospace A positive value of this function will give the occurrence probability of an increase of resistivity with respect to a reference resistivity value, whereas a negative value will give the occurrence probability of a decrease

of resistivity By scanning the tomospace, a full 3D image reconstruction of the anomaly sources distribution can at last

be obtained in a probabilistic sense

Zone B S7 S5

S6

Zone C S8 S10

S9 S4

S3

S2 S1

Zone A

N

Resistivity anomaly occurrence probability

Figure 11: Geoelectrical probability tomography at 1 m of depth

b.g.l and locations of the mechanical surveys S1-S10, indicated by

black circlets

Figure 12: A-Zone: the giacciara or icebox found in correspondence

of the rounded sequence of nuclei in the probability tomography of

Figure 11

A suitable reference resistivity can be either the true

background resistivity, if it is known, or simply the average

apparent resistivity, as we did in this study At the end

of the scanning procedure, one can draw sections or,

more efficaciously, 3D images of the probability distribution

pattern in the tomospace

Besides this primary scope of the method, worthy

of mention is a second, not less important peculiarity,

which makes the 3D probability tomography a versatile and

objective imaging approach Since the algorithm can deal

even with multiple datasets, independently of the acquisition

technique, it also works as an intrinsic filter The result is

a simultaneous smoothing of the uncorrelated noise and

suppression of any correlated phantom effects In principle,

this peculiarity derives from the circumstance that such types

of disturbances have zero probability to be generated by

true anomaly sources within the context of the geoelectrical theory

Concluding, we think that it is useful to point out that the probability-based ERT approach cannot give estimates of the true resistivity contrasts which characterise the sources

of anomalies Therefore, it appears to be more appropriate

in those circumstances in which the resistivity contrast of the targets is either known in advance or relatively less relevant than the discovery of their existence and retrieval of their shape This is usually the case in target-oriented applications

to archaeology Otherwise, the method can be considered a valuable support to the classical interpretation Its results can

in fact be used as a priori robust geometrical constraints in anyone of the inversion routines

4 The Survey Planning

As previously anticipated, the ERT survey was carried out

by a direct current multielectrode resistivity meter, capable

of handling up to 254 electrodes The DD electrode device was adopted using 1 m long dipoles displaced at a step of 1 m

along each profile with k in (1) reaching the maximum value

of 10 The following three different zones (Figure 4) were investigated

Zone A: it is an area of 1674 m2, located close to the northern side of the fortress, where a car park underground had been planned 37 parallel profiles, 31 m long and spaced 1.5 m apart, were realised Each profile consisted of 235 measurements, thus totalling 8695 data points In this zone,

a giacciara is indicated with a circle in the old drawing of the

fortress, close to its northern wing (Figure 2) The probable presence of the round structure, likely a brickwork room used in the past for the maintenance of food, is also suggested

by the cropmarks easily visible on the ground (Figure 5), nearly where it is indicated in the map ofFigure 2

Zone B: it covers an area of 620 m2, located south of the castle, where the construction of a swimming pool had been planned 21 parallel profiles, 31 m long and spaced

1 m apart, were investigated Each profile consisted of 185 measurements, thus totalling 3885 data points

Zone C: it covers an area of about 600 m2, located inside the southwestern portion of the ditch that surrounds the fortress, where part of the destroyed southern wing of the castle was founded Due to logistic difficulties, 18 profiles spaced 1 m apart, but with different lengths ranging between

15 m and 31 m, were measured The total number of data points was 3560

values inΩm obtained in each of the surveyed zones The mean value in each zone was calculated taking the average of the logarithms of the correspondingρ avalues

For the sake of brevity, we show in Figures6,7, and8

only three apparent resistivity pseudosections, one for each zone, in order to give an idea of the nature of the input data

We now proceed directly to the 3D probability tomography approach that we have used to detect and confine the sources

of the apparent resistivity anomalies

34 32 30

36

38

N

−4

−2 0

22 24 26 28

−4

−2

0

22 24

26 28 30

34 38

−4

−2

0

28 26 24 22 30 34 38

Resistivity anomaly occurrence probability Distances (meters)

(a)

34 32 30

36 38

N

22 24 26 28

2224 26 28

30 32 34 36 38 28 26 24 22 30 32 34 36

Resistivity anomaly occurrence probability

Distances (meters)

(b)

Figure 13: A-Zone: a sketch of the giacciara compared with the particular of the round sequence of nuclei extracted from the 3D image of

Figure 10(a), from a lateral (a) and a top (b) view

Table 1: The minimum, maximum, and mean apparent resistivity

values inΩm obtained in the surveyed zones A, B, and C

5 The 3D Probability Tomography

imaging applied to the{ ρ a } datasets collected in the three

zones marked inFigure 4 In each zone, the number of the

model cells used for the 3D ERT imaging is exactly the

same as that of the data points The 3D ERT image consists

of a sequence of horizontal slices at increasing depth from

1 m down to 5 m beneath the ground level (b.g.l.) A rather

complex pattern of resistivity anomaly sources can readily

be observed The group of sources, deserving to be analysed

from the archaeo-architectural point of view, can reasonably

be associated with the highs occurring within the first 3 m of

depth These highs, that is, source nuclei characterised by a

positive occurrence probability, would indicate the presence

of structures with true resistivity higher than the reference

uniform resistivity in each zone The large positive and

negative nuclei, centred at a depth not less than 4 m b.g.l in

the B-zone and the C-zone, respectively, and within which the maximum absolute occurrence probabilities have been obtained, may reasonably indicate, instead, the presence of

a vertical discontinuity This discontinuity is assumed to separate two geological media with resistivity on a side higher and on the other side lower than the reference resistivity The reference resistivity was taken equal to the average apparent resistivity of 29.92Ωm in the A-zone, 22.80 Ωm in the B-zone, and 87.04Ωm in the C-zone

More compact 3D views of the highs of probable archaeo-architectural interest under the surveyed zones are reported separately inFigure 10 In all of these and following 3D views the resistivity anomaly occurrence probability scale has been modified, by compressing the light-to-dark colour sequence entirely within the positive half-scale and leaving colourless the negative half-scale

Moreover, to help focus the discussion, we consider

as a reference map the horizontal slice at 1 m of depth, extracted from Figure 9 and depicted in Figure 11 All of the relative maxima of the highs of probable archaeo-architectural interest are located in this slice, where the sites

of 10 holes, bored after the ERT prospecting for ground-truthing, are also indicated The results of this activity are reported separately for each zone

5.1 A-Zone Worthy of note appears the isolated rounded

sequence of nuclei visible in Figure 11at the centre of the

Figure 14: B-zone: the furnace found in correspondence with the

longer side of the L-shaped sequence of nuclei in the geoelectrical

tomography ofFigure 11

A-zone, close to its right-hand borderline The location of

this source exactly corresponds with the cropmarks visible in

a circular structure with radius and height of 3.3 m to be

discovered (Figure 12), immediately under the humus [12]

It was found made of pebbles and bricks tied up with a

mortar rich in sand in the top portion, made of disjointed

bricks and slightly flared at the bottom, and externally

surrounded by eight small buttresses, set at a regular distance

of about 2.6 m This regular and well-preserved masonry

structure was readily ascribed to the circular plot indicated

as giacciara in the ancient drawing reported inFigure 2[12]

For a better appreciation of the resolving power of the

exposed probability tomography method,Figure 13shows a

zoom of the 3D image inFigure 10(a), under two different

angles of view, limited only to the central round sequence

of nuclei A sketch of the giacciara is also plotted at the

correct place as from the digging In both images, the round

sequence of nuclei appears to correspond exactly with the

trace of the giacciara on the horizontal plane through its

centre Furthermore, in the lateral view inFigure 13(a), the

full bowl-shaped set of source nuclei appears to smoothly

conform to the very regular nest-shaped structure of the

giacciara.

In the A-zone 4 holes (S1-S4) located as inFigure 11were

also bored The results from the S2 hole, bored down to 5 m

of depth in correspondence with the top right alignment of

small positive nuclei, showed the only significant anomalies

In S2, a layer with significant signs of human activity

was in fact detected at about 1.65 m of depth A

slimy-sandy-clayey layer, rich of bricks and carbonaceous frustules,

was found [13] This fertile layer has, however, not yet

been confirmed by direct archaeological excavations It may

reasonably extend over the whole top third of the A-zone, likely as patches separated by sterile zones A support to this interpretation derives from the discontinuous nature of the dark nuclei and the circumstance that the hole S1, located a little outside the large horizontal sequence of nuclei in the top left-hand side of the area, did not meet any remnants

No interesting archaeological data came also out of the S3 and the S4 holes, thus confirming the absence of resistivity anomaly source nuclei in those areas

5.2 B-Zone An inclined L-shaped sequence of positive

nuclei was the only interesting feature appeared in the B-zone, as shown in Figure 11 Three holes (S5-S6-S7) were bored The S5 hole, located on the longer side of the L-shaped source sequence, revealed, from 0.65 m down to 1.7 m b.g.l., the presence of residues testifying an activity of combustion and/or heating [13] The first 0.45 m resulted,

in fact, to belong to a brick structure, the following 0.15 m

to be made of ash and coal and the last 0.45 m of burnt clay The subsequent digging brought to the light a roofless furnace dating back to the 15th-16th century (Figure 14)

The complex presents two anterooms (prefurni) and a room

of combustion that probably contained part of the last batch

of bricks employed for the construction of the castle [12] The hole S6 was bored down to 5 m of depth, in correspondence with the small positive nucleus located at the centre of the lower half of the zone The most significant archaeological layer was found between 0.7 and 1.47 m of depth with abundant brick fragments In particular, the first strata are worthy of note because remnants similar to those found in the S5 hole were detected in the same depth range, allowing for a connection between them The S7 hole, instead, was bored down to 5 m of depth in an area where the geoelectrical tomography did not put in evidence any relevant positive nucleus The hole confirmed such a result, since only very rare fragments were there detected [13]

As before, a zoom of the 3D image inFigure 10(b), under two different angles of view, limited only to the portion of the longer side of the L-shaped sequence, where the nuclei with highest occurrence probability were found, is depicted

A sketch of the furnace is also plotted at the correct place as inferred from the digging In both 3D images, it can readily

be observed that the selected portion of the L-shaped set of nuclei exactly corresponds with the trace of the discovered furnace on the horizontal plane through its floor Moreover, the top view inFigure 15(b)shows that the sequence of the small nuclei closely conforms to the disposition and shape of the discovered rooms

5.3 C-Zone The C-zone is almost totally dominated by

a positive pattern of source occurrence probabilities, com-posed of a double set of parallel positive nuclei, which appear

to conform at right angle to the southwestern corner of the castle The internal sequence of nuclei may reasonably attest the presence underground of the foundations of the fourth wing, including the tower, as indicated in the drawing

sequence of positive nuclei may, instead, be associated to

125 115

−4

−2 0

30 34 38

0

30 34 38

125 120 115

0

38 34 30 125 120 115

Resistivity anomaly occurrence probability

Distances (meters)

(a)

125 120

115

30 34 38

30 34 38

125 120 115

38

34

30 125 120 115

Resistivity anomaly occurrence probability

Distances (meters)

N

(b)

Figure 15: B-Zone: a sketch of the furnace compared with the last portion of the longer side of the L-shaped sequence of nuclei extracted from the 3D image ofFigure 10(b), from a lateral (a) and a top (b) view

Figure 16: C-zone: a nearly east-westward view of the foundations

of the fourth wing of the Castle of Zena found in correspondence

of the inner sequence of nuclei at right angle in the geoelectrical

tomography ofFigure 11

traces of structural elements connected to the castle, likely

the base of the former embankment of the ancient ditch

In the C-zone three holes were bored The hole S8 was

bored down to the depth of 4 m outside the area surveyed

265 260 255 250

145 150 155 160 165 170

Resistivity anomaly occurrence probability

Distances (meters)

Figure 17: C-Zone: a sketch of the foundations of the destroyed fourth wing and southwestern tower of the castle compared with the inner sequence of the resistivity anomaly source nuclei, replicated fromFigure 10(c) The present outline of the castle is drawn on top

by the ERT At 1.75 m of depth, a compacted gravel layer was found with traces of mortar, probably constituting the

allurement layer of the fourth wing [13] The hole S9 was

bored down to the depth of about 2.5 m in the area of the

old ditch now turned into orchard Bricks were found from

0.97 m down to 1.10 m Finally, the hole S10 of 4.5 m of

depth did not reveal any elements referable to the destroyed

wing, but only levels related to the depositional activity

in the ditch [13], thus confirming the absence of positive

anomaly source nuclei in the ERT of that sector of the

C-zone The subsequent archaeological excavations (Figure 16)

brought to light the existence of the foundations of the

collapsed southern wing of the castle and the tower at

the southwestern corner [12] Also in this case, in order

to evaluate the resolving power of the 3D tomography

reconstruction, the 3D tomography image in Figure 10(c)

is replicated inFigure 17for a comparison with the plotted

sketch of the foundations as discovered by the digging

6 Conclusion

We have shown the results from an application of the 3D

probability-based ERT imaging approach to a case-study of

great importance from both the historical and architectural

points of view, consisting in the mapping of some structural

remains of the medieval Castle of Zena in three adjacent areas

destined to restoration and renovation

As outlined in previous applications [14, 15], the 3D

probability-based ERT method can be considered a

self-sufficient procedure, useful to delineate location and shape of

the most probable sources of the anomalies detected on the

ground surface In this study, and more generally in the field

of Cultural Heritage, the exact knowledge of the true

resistiv-ity is not so essential as the location and shape delineation

of the expected targets The rationale for this assumption

is that, generally speaking, buried stone remnants or metal

bodies of archaeo-architectural interest are characterised

by true resistivities higher or lower, respectively, than the

resistivity of the hosting environment, which normally

consists of medium-to-low resistivity sediments This, of

course, facilitates the detection of meaningful anomalies on

the measurement surface It must also be stressed that the

knowledge of the resistivity of the targets generally does not

give any added value to the immediate interest of the

histori-ans, archaeologists, or architects This is the reason why the

3D probability tomography approach has not required in this

application a further step, aimed at associating true resistivity

values to the revealed anomaly sources

The subsequent ground-truth excavations are to be

con-sidered a further successful confirmation of the full validity

of the 3D probability-based ERT imaging in detecting the

targets location underground and of its resolving power

For a more detailed discussion on this aspect and examples

of comparison with the results of inversion-based ERT

approaches, the reader is referred to [9,14,15]

Acknowledgments

The authors wish to thank for the fruitful collaboration the

Archaeology Service of Parma and Piacenza and the Institute

of Technologies for Cultural Heritage, National Research Council (CNR), Italy Thanks are also to Dr P Mancioppi, GEONORD Company, Piacenza, Italy, who has kindly pro-vided them with the stratigraphic results from the boreholes, and to Professor A Augenti and his group, Department of Archaeology, Ravenna branch, University of Bologna, Italy, who has supervised the archaeological excavations in the areas indicated by the geoelectrical probability tomography

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